1. Field of the Invention
The present invention relates to an optical reading-writing apparatus wherein laser beam is irradiated on a data recording medium such as a magneto-optical disc for writing, reading and erasing the data.
2. Description of Prior Art
In recent years, a magneto-optical disc is used as a high density and mass storage memory. The magneto-optical disc is used as a data recording medium in an optical reading-writing apparatus. The magneto-optical disc apparatus is required to be compact and simple and to have a performance of high-speed access.
A conventional optical disc reading apparatus used only for reading the data in a conventional optical reading-writing apparatus is described referring to FIG. 8.
In FIG. 8, an optical disc 1 as a data recording medium is rotated by a spindle motor 2. A semiconductor laser 3 is used as a light source for reading signals written on the optical disc 1. On an optical base 4, which is provided for slidably moving in radial direction of the optical disc 1, a semitransparent mirror (beam splitter) 5 for splitting the light beam from the semiconductor laser 3 in two ways and mirrors 6 and 7 are provided. The light reflected by mirror series of semitransparent mirror 5 and mirrors 6 and 7 is converged on a predetermined position on the optical disc 1 by an objective lens 8. The light beam reflected by the surface of the optical disc 1 is further converged by objective lens 8, reflected by the mirrors 7 and 6. And the light passes through the semitransparent mirror 5 and reaches to a photo-sensor 9.
A feed screw 10a and a guiding rod 10b are provided in radial direction of the optical disc 1. And the optical base 4 is driven in the radial direction shown by arrows K by rotation of the feed screw 10a.
In the above-mentioned conventional optical disc reading apparatus, a high-speed access can not be achieved since the optical base 4 is driven by the feed screw 10b. For acceding in a high-speed, a linear motor constituted by a driving coil and a magnetic circuit is proposed for replacing the feed screw 10 for driving the optical base 4. In such use of the linear motor, the optical base 4 can be driven in a high speed in radial direction of the optical disc 1 by the driving coil and magnetic circuit in principle. The optical base 4, which, comprises optical elements such as the semiconductor laser 3, the photo-sensor 9 and so on, however, has a large mass, thereby hindering sufficient improvement of the access speed.
Furthermore, when a magneto-optical disc is used as a data recording medium, a magnetic head for over-writing is necessary to be fixed on an U-letter shaped optical base for configurating a magneto-optical over-writing moving unit of a magnetic modulation type. In this case, mass of the optical base is further increased and the access speed becomes slower.